Because a heat-developable photosensitive material comprises therein an oxidizing agent and a reducing agent for an image-formation, simultaneously, the photosensitive material has such a disadvantage that fog is apt to increase during preservation. In addition, in most cases, a heat-developable photosensitive material contains a base or base precursor therein, for the purpose of acceleration of development under heat. In general, base precursors capable of releasing a basic substance by pyrolysis are preferably used, to improve the preservability of the photosensitive materials themselves.
However, many base precursors are often accompanied by noticeable fog during development. Such base precursors often partially decompose, during storage before use, resulting to deterioration of photographic characteristics of the photosensitive materials, and particularly in a remarkable increase of fog therein. Under the circumstances, some further improvement is particularly required in materials containing such base precursors.